What happens next is up to the logic of the server sending the mail. It'll try sending the messages to the next in the MX priority list - which for google-handled domains are going to be the same. And again, all but one of the delivery domains will get rejected.

I've yet to see a mail bounce as a result of this, but for mails with many different domains I've seen deliveries hit their retry limits and hang around on the mail queue for over an hour. Sub-optimal. A web-search for "Multiple destination domains per transaction is unsupported" will likely locate a few annoyed mail-admins.

You can work around this in Exim. Exim has a transport option "multi_domain" that, when set to false, prevents multiple domains from being delivered per transaction. So you need to configure mail to route all google-handled domains via a transport that has this set.

First, set-up a new transport called "remote_smtp_single_domain" - this should be the same as your existing remote_smtp transport, but with "multi_domain = false" /etc/exim4/conf.d/transport/40_temp_single_domain